using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<UserDetails> userIds=new List<UserDetails>{new UserDetails{UserIds="1",Type="a"},new UserDetails{UserIds="2",Type="b"}};       

        UserDetails[] obj=userIds.ToArray();

        var output = "users/outboundUserdetails?userIds=" + string.Join("&userIds=", userIds);

        Console.WriteLine(output);

    }

}

public class UserDetails

    {

        public string UserIds { get; set; }

        public string Type { get; set; } // Inbound or outbound

    }